1. Field Of The Invention
The present invention relates to a combined cycle electric power plant and more particularly to improved fluid level control for heat recovery steam generators and condensers associated therewith, as particularly adapted for use in combined cycle electric power plants.
2. Description Of The Prior Art
In the design of modern electric power plants, it is a significant object to achieve the greatest efficiency possible in the generation of electricity. To this end, steam generators are designed to extract heat efficiently from and to use the extracted heat to convert a fluid such as water into superheated steam at a relatively high pressure. Further, such a steam generator has been incorporated into combined cycle electric power plants including both gas and steam turbines wherein the exhaust gases of the gas turbine are used to heat water into steam then to be transferred to the steam turbine. Further, a condenser is associated with the steam turbine for receiving the spent steam and converting it into a condensate to be supplied to the steam generator.
Typically, a steam generator comprises a deaerator for receiving the condensate from the condenser and includes a storage portion, a feedwater pump for directing the fluid stored inthe storage portion to a first or economizer heat exchanger, a high pressure evaporator, and finally a superheater tube, whereby water is gradually heated while increasing levels of pressure are applied thereto to provide from the superheater tube, superheated steam to be supplied to the steam turbine. The steam generator also includes a main drum for receiving and storing the heated fluid before being directed therefrom to be further heated by one of the heat exchange tubes. The heated exhaust gases derived from the gas turbine are used to heat the fluid passing through the heat exchange tubes of the heat recovery steam generator. In this manner, a significant reduction in the fuel required to heat the steam is achieved and the heat contained in the gas turbine exhaust gases is effectively utilized.
A problem occurring in the prior art relates to the possible damage of the feedwater pump used to direct fluid from the deaerato to the first heat exchange tube. In particular, as the relatively cold condensate is introduced into the deaerator and in particular to its storage portion, the pressure therein tends to drop rapidly, whereby the pressure exerted upon the feedwater pump may be sharply reduced. If the pressure is suddenly removed from the feedwater pump, it may begin to cavitate with possible damage occurring thereto.
This problem is especially significant where it is not feasible to dispose the deaerator and in particular its storage tank, a sufficient level above the feedwater pump so as to establish a head of fluid thereon, whereby pressure would normally be exerted upon the feed pump. This situation arises in the electric plant as described in the above-identified application entitled "Combined Cycle Electric Power Plant Having A Control System Which Enables Dry Steam Generator Operation During Gas Turbine Operation", which may be constructed in a modular fashion whereby each element of the entire system is disposed within a housing and shipped to the plant site, whereat it is assembled upon concrete slabs. As a result of the modular construction, it is not feasible to space the deaerator and its storage portion a significant distance above the feedwater pump. As a result, if there is experienced a significant reduction in the pressure within the storage portion, the pressure would be rapidly removed from the feedwater pump with possible damage resulting thereto.
In the steam generators of the prior art, it is typical to use a three-element control whereby the storage level within the storage portion of the deaerator is maintained at a normal preset value. The measured parameters are the condensate flow to the deaerator, the feedwater flow from the storage portion and into the main drum, and the fluid level within the storage of the deaerator. By the use of such control, the condensate flow into the storage portion of the deaerator is made to follow the feedwater flow out. This causes a particular problem when there is a sudden drop in the demand for superheated steam, whereby the flow of steam from the main drum is reduced sharply. However, the feedwater into the main drum and out of the storage portion of the deaerator is maintained at a relatively high level. Thus, in the prior art, a high rate of condensate flow is maintained into the deaerator storage portion as by regulation of the condensate valve, whereby a relatively high mass of cold fluid is introduced with a subsequent drop in pressure within the deaerator storage portion. As mentioned above, this may cause significant damage to the coupled feedwater pump.
The description of prior art herein is made on good faith and no representation that any prior art considered is the best pertaining prior art nor that the interpretation placed on its is unrebuttable.